An optical fiber (e.g., optical fiber conforming to ITU-T G.652 standard) including a core made of silica glass containing GeO2, which is a dopant for increasing the refractive index, is referred to as a “GeO2-doped silica core optical fiber” in this Description. An optical fiber including a core made of substantially pure silica glass without containing a dopant for increasing the refractive index (e.g., GeO2 and Al2O3) is referred to as a “pure silica core optical fiber” in this Description. The pure silica core optical fiber has low transmission loss and good long-term reliability such as hydrogen resistance and radiation resistance compared with the GeO2-doped silica core optical fiber (refer to Y. Chigusa, et al., J. LIGHTW. TECHNOL., VOL. 23, NO. 11, pp. 3541-3550, 2005). The GeO2-doped silica core optical fiber is sometimes exposed to deuterium gas (D2) under certain conditions in order to improve the hydrogen resistance (refer to JP2003-261351A). On the other hand, the pure silica core optical fiber has good hydrogen resistance and thus the exposure to deuterium gas is generally not required.
When an optical fiber is produced by drawing an optical fiber preform, a heating furnace is disposed near the downstream of a drawing furnace. An optical fiber immediately after the drawing process is caused to pass through the heating furnace, and the optical fiber is heated to a predetermined temperature range when passing through the heating furnace. By reheating the optical fiber immediately after the drawing process by the heating furnace, the optical fiber after the drawing process is prevented from being rapidly cooled and thus is slowly cooled. It is known that the relaxation of the network structure of glass due to rearrangement of atoms is facilitated when the optical fiber is heated in the heating furnace, which decreases the fictive temperature Tf in the optical fiber and reduces the Rayleigh scattering intensity in the optical fiber, resulting in a reduction in the transmission loss of the optical fiber (refer to JP4482955B, JP 4482954B, JP435615513, JP4356154B, JP 2002-148465A, JP4400026B, JP4155001B, JP4244925B, JP4124254B, JP4741251B, JP 2011-505326A, and U.S. Pat. No. 7,876,990B).
In order to reduce the transmission loss of pure silica core optical fibers to, for example, 0.155 dB/km or less at a wavelength of 1550 nm by using such a heating furnace, the relaxation of the network structure of glass needs to be further facilitated by lengthening the heating furnace to several tens of meters or decreasing the drawing speed (speed at which an optical fiber is drawn from a drawing furnace) to several tens of meters per minute. However, in the former, it is difficult to realize such optical fiber production facilities. In the latter, the productivity considerably degrades.
“Alkali metal-doped silica core optical fibers” are known as optical fibers in which Rayleigh scattering intensity can be decreased (refer to JP2005-537210A, US2006/0130530A, JP2007-504080A, JP 2008-536190A, JP 2010-501894A, JP 2009-541796A, JP 2010-526749A, WO98/002389, and U.S. Pat. No. 5,146,534B). Alkali metal-doped silica core optical fibers are optical fibers including a core made of silica glass containing an alkali metal (e.g., Na and potassium) in a trace amount of 500 ppm or less without containing a dopant for increasing the refractive index, such as GeO2 or Al2O3. If a core portion of an optical fiber preform contains an alkali metal, the viscosity of the core portion can be decreased when the optical fiber preform is drawn, and thus the network structure of silica glass is relaxed. Therefore, it is said that the fictive temperature Tf in the optical fiber decreases, which can reduce the transmission loss. In the core of alkali metal-doped silica core optical fibers, not only the amount of an alkali metal added, but also the amount of a halogen such as Cl or fluorine added is a trace amount.